Projects/Nepomuk/OntologyBasics

Nepomuk unlike other conventional projects, does not use a relational database to store its data. Instead it uses a RDF store. RDF stands for Resource Description Framework. It's the big framework that was written for describing abstract data on the web, and is the basis of the semantic web. While RDF, has many concepts, Nepomuk mainly utilizes a couple of them.

Resources

Statements

The simplest concept is that of a sentence. All the data in Nepomuk is stored in the form

<subject> <predicate> <object> <graph>

These 4 values combined are called a statement.

If we look at this in terms of an object oriented programming language, the `<subject>` is an OOP Object, and the (predicate, object) pair can be looked as (key, value) pairs which define properties on that OOP Object.

Subject

The subject, like in conventional english, is the main part of the sentence. It highlights what we are talking about. In the above example there are different subjects nepomuk:/res/some-uuid and nepomuk:/res/some-other-uuid. In each of these statements the subjects have certain properties and objects bound to them.

Predicate

Predicates are often also called properties, and they used to connect the <subject> with the <object>.

Each predicate is generally assigned a domain and range which governs the kind of <subject>s and <object>s it can be connected to.
Some examples of commonly used predicates are rdf:typenao:lastModified and nao:hasTag

Object

Graph

The graph contains some information about the triple - (<subject> <predicate> <object>) such as when it was created, and last modified. Unless you're working on the absolute core parts of Nepomuk, you won't ever have to interact with graphs, and they'll just work seamlessly in the background.